Depending on how heavy they are and how far you have to travel you can. If their rather heavy I would wait until the cake is at its destination. Car movement and gravity are not on your side so if your in doubt that they wont make it, its better to be safe than sorry!

Thanks for the comments. The flowers are small and are going on the sides of the cake which will be iced in a crusting buttercream. The reception is at our church, which is where the cakes will be decorated, so thankfully they won't need to be transported after they are decorated.

As long as they're small, they should be just fine. My gumpaste flowers don't soften up overnight when I put them onto the buttercream. However, if they're too heavy they sometimes slide down. Gravity is not our friend!!